Different ways to manufacture such coils are known. For example, U.S. Pat. No. 5,525,850A describes an axial coil for a slot-less motor. This coil is winded with electrical conductors parallel to the rotation axis, and in the two coil heads the wires are winded back after an angular displacement to provide a uniform interior diameter extending along the length of the stator for receiving the rotor of the motor. This coil presents the disadvantage of requiring a relatively long axial length for the heads. This has a negative impact on the total length of the motor and consequently increases the weight and the costs of the material needed to manufacture the whole motor frame and the rotation axis. The performances of the motor are also consequently affected since increased mass reduces the acceleration possibilities. Another source of efficiency loss is the high electrical resistance of this coil, as it is required to have a higher length of electrical conductor so that the overall electromagnetic efficiency is affected. It may also be noticed that the electrical conductors used, even with an optimized winding process, will not occupy the whole space available and there will be free space between the winded wires. The conductor filling factor is the ratio of electrical conductor volume to the total volume of the coil. The use of such conductors leads to a poor conductor filling factor, which hinders the realization of a low-length and low-diameter motor.
Some attempts have been carried out to reduce volume of the coil heads. In particular, the document JP1252135A discloses coils with three different winding arrangements of the electrical conductors to reduce the volume of the coil heads. In two of the arrangements, the winding of the conductors is done with an angular displacement of the conductor in the coil body so as to reduce the size of the coil heads. However, such coil winding has the disadvantage of reducing the torque and power factors of the motor, as the electrical conductors are not parallel to the rotation axis, but rather extend at a significant angle to the rotation axis, resulting in a reduction of the efficiency of the electromagnetic forces.